I bought iPhone 15 pro and the battery doesn’t last a half a day before I have to charge

Well, Battery Health is going to decline on average at about 1% a month, so plan on and expect that. But, that's a average. Some months, it will not decline at all, then drop 2% the next month.

As far as holding a charge, you need to make an appointment at the Apple Store or take the phone to an authorized Apple service shop to let them run some checks and tests.

You can find a list of authorized Apple service shops in your area using this link:

Find Locations

To maybe save you a trip to an Apple Store, have a look at this information:

Check Settings > Battery feature to see if there are apps or services running that use a lot of power. If there are, decide if you need them running all the time, can turn off a service when an app is in the background, or disable the feature completely. Read this support article to learn about the battery health feature. —> iPhone Battery and Performance - Apple Support

Refer to this post for good tips on reducing battery use on your device --> https://discussions.apple.com/thread/255063652?answerId=259418848022#259418848022

Addendum to #11. Use 4G/LTE instead of 5G. Use 5G with your iPad - Apple Support

Here is a related post in the context of device heating. --> https://discussions.apple.com/thread/255063343?answerId=259418333022#259418333022